The present invention relates to the field of fiber optic transmission and in particular to repeaterless transmission systems. It relates in particular to wavelength division multiplex (WDM) fiber optic transmission systems.
Optical amplifiers, and in particular erbium-doped optical fiber amplifiers, are provided at regular intervals along one type of prior art system for transmitting signals in optical fibers. A solution of this kind is described in Bergano, xe2x80x9cLong haul WDM transmission using optimum channel modulation: 32xc3x975 Gbit/s 9300 km demonstrationxe2x80x9d, OFC""97 post deadline 16, for example. The transmission distances in a system of the above kind are limited by the signal-to-noise ratio and in particular by amplified spontaneous emission (ASE) generated in the amplifiers.
The article by Morten Nissov et al, xe2x80x9c100 Gb/s (10xc3x9710 Gb/s) WDM transmission over 7 200 km using distribution Raman amplificationxe2x80x9d, OFC""97, post deadline paper, proposes the use in a transmission system of only distributed stimulated Raman scattering (SRS) amplification to amplify the signal at regular intervals. This solution improves the signal-to-noise ratio by about 2 dB compared to a comparable solution using only discrete pumps. A description of the Raman effect is given in xe2x80x9cNonlinear Fiber Opticsxe2x80x9d by G. P. Agrawal, Academic Press, 1980.
P. B. Hansen et al., in xe2x80x9cUnrepeated WDM Transmission Experiment with 8 Channels of 10 Gb/s over 352 kmxe2x80x9d, IEEE Photonics Technology Letters vol. 8, no. 8, Aug. 1996, pages 1082-1084, describe a repeaterless transmission system with remote pumping of erbium-doped fiber sections of the transmission system at a distance from the transmitter and the receiver and pumped by sources respectively in the receiver and in the transmitter.
French Patent Application 98 16496 filed Dec. 28, 1998, whose title in translation is xe2x80x9cQuasi-distributed amplification in a fiber optic transmission systemxe2x80x9d, proposes a repeater for a fiber optic transmission system which includes a discrete amplifier, for example an erbium-doped fiber amplifier, with at least one pump for producing amplification in the line fiber by stimulated Raman scattering.
One problem encountered with Raman amplification concerns the wavelength of the pumps employed. This problem arises in particular in Raman amplification of wavelength division multiplexed signals which extend over a wide range of wavelengths. This problem is mentioned by S. V. Chernikov et al. in xe2x80x9cBroadband Raman amplifiers in the spectral range of 1480-1620 nmxe2x80x9d, OFC""99 m, WG6, pages 117-119. The solutions proposed in the above document are to cascade amplifiers having different pump wavelengths or to couple pumps at multiple wavelengths into a single amplifier.
However, the above document does not describe any way of coupling pumps at multiple wavelengths into a single amplifier.
The invention proposes a simple and effective solution to the above problem which can be implemented using a limited number of standard optronic components to provide continuous and uninterrupted amplification in the wanted band.
To be more precise, the invention proposes a system for injecting into a fiber signals from multiple sources at different wavelengths, the system comprising:
a first signal source,
a first circulator with a first input connected to the first source,
a second signal source, and
a second circulator with a first input connected to the second signal source via reflector means for reflecting signals from the first source, a second input supplying the signals from the first and second signal sources, and a third input connected to a second input of the first circulator.
One embodiment of the system further includes:
a third signal source, and
a third circulator with a first input connected to the third signal source via reflector means for reflecting signals from the first source and the second source, a second input supplying the signals from the first, second and third signal sources, and a third input connected to the second input of the second circulator.
In this case the system can further include:
an nth signal source, and
an nth circulator with a first input connected to the nth signal source via reflector means for reflecting signals from sources of rank less than n, where n is an integer varying from 4 to M and M is the total number of sources injected, a second input supplying the signals from the signal sources of rank 1 to n, and a third input connected to the second input of the circulator of rank nxe2x88x921.
Each of the signal sources advantageously comprises a pump.
In one embodiment, the reflector means comprise Bragg gratings.
The invention also proposes an amplifier comprising a system as defined above for injecting pump signals and an amplifier fiber connected to the second output of the highest rank circulator.
The amplifier fiber is preferably a line fiber.
In another embodiment the amplifier fiber is not a line fiber.
Amplification is advantageously effected in the amplifier fiber by stimulated Raman scattering.
The invention finally proposes a fiber optic transmission system comprising an amplifier as defined above.